Electronic balance with load placement deviation correction

ABSTRACT

An electronic balance includes a pan, a first means for measuring a load normally applied to the pan, a second means for detecting a possible deviation of the load acting point from the required point on the pan, a third means for correcting an error in the measured value due to the deviation, and a fourth means for outputting the corrected value.

BACKGROUND OF THE INVENTION

The present invention relates to an electronic balance capable ofcorrecting an error in the measured value, the error being due to theturning moment caused by a load being placed deviatedly from a requiredpoint of placement, such as the center of the pan.

In general, when a load is placed deviatedly from a required point ofplacement, such as the center of the pan, which will be hereinafterreferred to as "eccentrically placed", the turning moment M exterts onthe pan, thereby producing an error E in the measured value. Anadjustment will be required for the geometrical precision of theRoberval mechanism of the balance. If the Roberval balance has asufficiently rigid construction to receive a load without any strain,the relations between the turning moment M and the possible error E willbecome linear. FIG. 1 (I) shows every possible situation of therelations therebetween, in which the lines A and B show opposite extremesituations. A skilled operator could produce an optimum situation Swithin an allowable range δ. However, this requires a high degree ofskill and experience. For example, when a Roberval balance has aprecision of 1/10⁶ to 1/2×10⁶, the adjustment must be made in the rangeof 0.1 to 0.05μ. On the contrary, if the Roberval balance does not havea sufficiently rigid construction, and receives a load with some degreeof strain, the relations between the turning moment M and the possibleerror E cannot be linear but vary as shown in FIG. 1 (II), which showsevery possible situation of the relations therebetween. Curves C and Dshow opposite extreme situations between which all possibilities canexist. However, in this case the most skilled operator could not producean optimum situation T within an allowance range δ₂. FIG. 1 (II) is acharacteristic graph for an extremely high precision balance, and theallowable range δ₂ is strict enough to admit of no negligence.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention is directed toward solving the problems pointedout above, and has for its object to provide an improved electronicbalance allowing of an easy and precise adjustment of an erroneousmeasured value due to the turning moment caused by an eccentric ordeflected placement of a load on the pan.

Another object of the present invention is to provide an improvedelectronic balance dispensing with the necessity of geometricalmanoeuvring adjustment, and allowing of electrical adjustment merely byoperating an electric adjuster such as a variable resistor.

A further object of the present invention is to provide an improvedelectronic balance having its weight measuring section rigidlyconstructed and its load deviation detecting section resilientlyconstructed, thereby increasing the sensitivity of load deviationdetection.

According to the invention, an electronic balance includes a pan, afirst means for measuring a load normally applied to the pan, a secondmeans for detecting a possible deviation of the load acting point from arequired point of placement on the pan, a third means for correcting anerror in the measured value due to the deviation, and a fourth means foroutputting the corrected value.

A measured value W obtained by a measuring means, measured values f_(X)and f_(Y) as component forces obtained by a deflection detector, thevalues f_(X) and f_(Y) being respectively for the X-direction and theY-direction, and a corrected value F(W, f_(X), f_(Y)) obtained by acorrecting means have been found to satisfy the following equation,

    F(W,f.sub.X f.sub.Y)=W+F(f.sub.X)+F(f.sub.Y)               (1)

where,

    F(f.sub.X)=K.sub.1X ·F.sub.X +K.sub.2 ·f.sub.X.sup.2 +K.sub.3 ·f.sub.X.sup.3 + . . .

    F(f.sub.Y)=K.sub.1Y ·f.sub.Y +K.sub.2Y ·f.sub.Y.sup.2 +K.sub.3Y ·f.sub.Y.sup.3 + . . .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more particularly described by way of example withreference to the accompanying drawings, in which:

FIG. 1 (I) and (II) are characteristic graphs showing relations betweenthe turning moment M and an error E in the measured value;

FIG. 2 is a schematic front view showing an electronic balance accordingto the invention;

FIG. 3 is a modified version of the embodiment;

FIG. 4 is a cross-sectional view taken along the line IV--IV in FIG. 3;

FIG. 5 is a further modified version of the embodiment in which aphoto-electrical system is schematically shown;

FIG. 6 (A) and (B) are cross-sectional views showing an arrangement ofsensors employed in the embodiment of FIG. 5;

FIG. 7 is a still further modified version of the embodiment in whichstrain gauges are employed;

FIG. 8 is a cross-sectional view particularly showing an arrangement ofthe strain gauges employed in the embodiment of FIG. 7;

FIG. 9 is an electric diagram employed in the embodiment of FIG. 7;

FIG. 10 is an electric block diagram of the electronic balance of theinvention; and

FIG. 11 is another modified version of the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2, a pan 1 is supported by a Roberval mechanism 3,which includes a measuring section 4 adapted to measure a load normallyapplied thereto. The pan 1 and the Roberval mechanism 3 are connected bymeans of a support 12 to which a deflection detector 2 is secured so asto detect a possible deflection occurring in the support 12 due to theturning moment exerting on the pan 1. The support 12 is connected to theRoberval mechanism 3. As is well known, the turning moment is caused bya load 11 being eccentrically placed on the pan 1, which does not alwaysmean that the load 11 is placed deviated from the center of the pan 1.It can mean that the load 11 is placed out of a required point ofplacement.

Functionally, the measuring section 4 measures the weight of a loadplaced on the pan 1, and outputs the measured value W. Structurally, itcan be any system selected from an electromagnetic balance, a load celladapted to detect a deflection occurring on a resilient or elastic bodyand signal it, or any other known systems.

The deflection detector 2 detects a deflection as its name implies,wherein the deflection is divided into two component forces (hereinafterreferred to as a component) crossing each other at right angle. One ofthe components is outputted as a signal f_(X) proportional to a degreeof the turning moment in the X-direction, and the other component is assignal f_(Y) proportional to a degree of the turning moment in theY-direction.

Structurally, the deflection detector 2 can be any system selected fromthe following examples:

One example is shown in FIG. 3, which utilizes a static capacity wherebya deflection due to the turning moment is sensed. In this embodiment thesupport 12 has a rectangular cross-section as best shown in FIG. 4. Thesupport 12 is surrounded by four electrodes 13X₁, 13X₂, 13Y₁ and 13Y₂each of which is located adjacent to each face of the support 12,whereby four capacitors are constituted. A possible change in the staticcapacity is detected in a known manner, such as through a change inresonance frequency in an LC resonance circuit or through a change inthe output voltage in the capacity detector circuit. When the support 12declines in any direction, a pair of opposite capacitors have differentcapacities; one has an increased capacity whereas the other has adecreased capacity. This means that the deflection is doubly detected orthat it is detected differentially with a doubly increased sensitivity.In FIG. 3 the support 12 is provided with a narrowed neck 14 designed toincrease the resiliency of the support 12, thereby making the support 12more responsive to the turning moment.

Referring to FIGS. 5 and 6(A) and (B), a photo-electrical method fordetecting a possible deflection will be explained:

In this embodiment the support 12 has a pair of branches 15 on which thepan 1 is supported. Between the branches 15 there is provided a shutter16 adapted to cut off the light from a light emission element 17 locatedabove the support 12. Four light sensors 18a, 18b, 18c and 18d arelocated oppositely to the shutter 16, wherein the sensors are fixedadjacent to each other. The shutter 16 shown in FIG. 6(A) has a hole 19produced in such a manner as to allow a substantially equal amount oflight to be given to each sensor when no load is applied to the pan 1.The sensors 18a and 18b, and 18c and 18d are arranged in theX-direction, and detect a displacement of the shutter 16 in theX-direction, which is inputted to a differential amplifier. The sensors18a and 18c, and 18b and 18d are arranged in the Y-direction, and detecta displacement of the shutter 16 in the Y-direction, which is alsoinputted to the differential amplifier.

In this way the movement of the shutter 16 in the X- and Y-direction isphoto-electrically detected. In a modified version shown in FIG. 6(B)the round hole 19 is replaced by two slits. The sensors 18a and 18bdetect a displacement of the shutter 16 in the X-direction, and thesensors 18d and 18c detect a displacement of the shutter 16 in theY-direction.

A further version of the embodiment is shown in FIGS. 7 and 8, which isbased on the principle of a strain gauge. The support 12 has four sides,and is additionally provided with a narrowed neck 20 in its middleportion. In each face two pairs of strain gauges 21a₁, 21a₂ : 21b₁, 21b₂: 21c₁, 21c₂ : 21d₁, 21d₂ are respectively provided. As shown in FIG. 9,a bridge circuit is constituted by the four strain gauges a₁, a₂, c₁,and c₂, which are provided on the faces crossing the X-direction, andanother bridge circuit is constituted by the strain gauges b₁, b₂, d₁,d₂, which are provided on the faces crossing the Y-direction. In thisway signals f_(X) in the X-direction and signal f_(Y) in the Y-directionare outputted.

Referring to FIG. 10, an electric diagram used for the invention will beexplained:

The reference numerals 31 and 32 designate deflection detector elementsfor the X-direction and the Y-direction, respectively. Outputs f_(X) andf_(Y) therefrom are amplified by amplifiers 33 and 34, respectively.These amplifiers 33 and 34 have gain adjusters including variableresistors. The resulting outputs are respectively K_(1X) ·f_(X) andK_(1Y) ·f_(Y).

As referred to above, the measured value W is outputted from themeasuring section 4.

An analogue switch 35 is provided to enable a single analogue-digitalconverter 36 to convert signals from the three different circuits intodigital form. The analogue switch 35 is quickly switched from one toanother under the control of a CPU.

A coefficient setting unit 37 is designed so as to enter compensationcoefficients K_(2X), K_(2Y), K_(3X), K_(3Y) . . . of the second andhigher order. The entries are performed by means of a numbered wheelwhereby a desired number is set, and the set numbers are displayed by adigital indicator. Alternatively, the entries can be made by means ofnumbered keys (ten keys) in association with the use of a RAM.

A computer 38 includes a CPU 40 having an input/output interface 39, aclock generator, and an arithmetic operator: a RAM 41 for temporarilystoring the measured data and the set values: a ROM 42 for storingprograms and equations: an indicator 43 for indicating the arithmeticresults: a digital output interface 44 for outputting the arithmeticresults outwards: an operating switch 45 for instructing a tarededuction and inputting the frequency of averaging of use, a unitconversion ratios and compensation coefficients: and a bus line 46 forconnecting all these elements.

A typical example of the operation will be explained:

If the load 11 is eccentrically placed on the pan 1, the turning momentwill result, which is detected as two components for the X-direction andfor the Y-direction. The operator determines the first ordercompensation coefficients K_(1X) and K_(1Y) for each direction byadjusting the variable resistor in the amplifiers 33 and 34, whereby thedata K_(1X) ·f_(X) and K_(1Y) ·f_(Y) corrected to the first order areinputted to the computer 38. As referred to above, the computer 38 isstoring secondary and higher order compensation coefficients, that is,K_(2X), K_(2Y), K_(3X), K_(3Y) . . . , and by referring to these higherorder corrected values, the measured value W obtained from the measuringsection 4 is processed. Then the corrected value f(W, f_(X), f_(Y)) isderived from the equation (1), and the corrected value is outputted.

As a result, the curves C and D in FIG. 1 (II) are corrected andstraightened, and are placed into the allowable range δ₂ like the line Sin FIG. 1(I).

Referring to FIG. 11, a further modified embodiment will be explained:

The illustrated block diagram shows the basic principle underlying thisembodiment, that is, a deflection is detected as components. Thisversion is particularly adapted for use in large-scaled electronicbalances.

A pan 51 is supported by two supports 52 and 53; the support 52 providesa force acting line to a lever 56 resting on a fulcrum 54, and thesupport 53 provides a force acting line to a lever 57 resting on afulcrum 55. The two levers 56 and 57 are connected to each other bymeans of a pin 58, and are connected to a load measuring section 59.Each of the supports 52 and 53 is provided with a load measuring scale60 and 61 respectively, which are designed to measure the degree of apossible deflection occurring when a load is eccentrically placed on thepan 51, wherein the deflection is detected as components. The scales 60and 61 do not call for a highly precise construction, but desirably,they can measure forces acting both in the plus and in the minusdirection, because when the load is placed at an extreme end portionoutside the point (A) (or (B)) a force acts in the minus direction onthe point (B) (or (A)). For the scales 60 and 61 strain gauges can beemployed.

Let the components be W_(A) and W_(B), and the distances from a loadacting point up to the points (A) and (B) be l_(A) and l_(B),

    W.sub.A +W.sub.B =W                                        (2)

    W.sub.A ·l.sub.A =W.sub.B ·l.sub.B       (3).

By these equations the load acting point can be located.

In FIG. 11 the scales 60 and 61 can be provided at the places indicatedby the reference numerals 54 and 55. In this case, a larger force willact at the point (A) or (B) at a ratio depending upon the lever ratio.This increment must be corrected. This embodiment has an advantage thatlead lines can stably and safely be located in a fixed portion of theapparatus.

What is claimed is:
 1. An electronic balance comprising a pan forreceiving a load to be weighed, a first means for measuring said load onsaid pan, said means being adapted to measure a load normally acting onsaid pan, a second means for detecting a possible deviation of the lineof action of said load from the required point of placement on said pan,a third means for correcting an error in the measured value inaccordance with a detected degree of deviation, and a fourth means foroutputting said corrected value.
 2. An electronic balance as defined inclaim 1, wherein said deviation is detected as a deflection occurring onthe support of said pan due to the turning moment, wherein saiddeflection is divided into two components crossing each other, andwherein said second means comprises two detectors, one being for theX-direction and the other being for the Y-direction.
 3. An electronicbalance as defined in claim 1, wherein said second means comprises oneelectrode movable in association with said pan and another locatedopposedly but spacedly to said electrode, thereby detecting changes inthe static capacity between said pair of electrodes.
 4. An electronicbalance as defined in claim 1, wherein said second means comprises aphoto-electrical unit including sensors and a light emission elementwith a shutter being interlocated therebetween, said shutter beingmovable in association with said pan and having an opening in itscenter, said opening allowing an equal amount of light to be given toeach of said sensors when no load is applied to said pan.
 5. Anelectronic balance as defined in claim 4, wherein said opening in saidshutter is a round hole.
 6. An electronic balance as defined in claim 4,wherein said opening in said shutter is a slit.
 7. An electronic balanceas defined in claim 1, wherein said second means comprises a strainsensor provided in a support of said pan, said support being movable inassociation with said pan, thereby enabling said strain sensors todetect a possible deflection occurring in said support.
 8. An electronicbalance as defined in claim 1, wherein said first means comprises aRoberval mechanism connected to said pan by means of a support, saidsupport including a narrowed neck designed to impart an increasedresilient nature to said support, and wherein said second means hassensors located between said pan and said narrowed neck in said support.9. An electronic balance as defined in claim 1, wherein said secondmeans supports said pan at plural points, and wherein said first meansis provided at each of said points, thereby measuring a possibledeflection through the measurement of divided forces acting on each ofsaid points.
 10. An electronic balance as defined in claim 1, whereinsaid third means comprises a digital logical operation unit, a gainadjusting means located between an analogue output line of said secondmeans and an input line of said digital logical operation unit, andmeans for entering an operation coefficient to said digital logicaloperation unit.